1. Field of the Invention
This invention relates to a device for measuring ionic concentration or ionic activity, and more particularly to an ionic activity measuring device useful for potentiometric measurement of the concentration or ionic activity of an ion contained in specimens such as water, body fluids (for example, whole blood, blood plasma, blood serum, urine and the like), and aqueous solutions (for example, service water, factory waste water, river water, rain water, wine, beer and the like).
2. Description of the Prior Art
Generally, from the clinical or industrial point of view, it is important to selectively measure the concentration or the ionic activity of an inorganic ion, for example K.sup.+, Na.sup.+, Ca.sup.2+, Cl.sup.- or HCO.sub.3.sup.-, contained in body fluids or aqueous solutions. For this purpose, it has been proposed to use a wet type ion selecting electrode.
The conventional wet type ion selecting electrode is in the form of a needle or a bar and dipped in a specimen such as body fluid each time it is used for measurement. Therefore, the electrode of this type must be appropriately maintained, washed and conditioned after it is used for measurement. This adversely affects the service life of the electrode and causes the electrode to break easily, requiring troublesome electrode management and great expense. Furthermore, it is necessary to use a large amount (e.g. several hundreds of microliters or more) of a specimen for measurement because the electrode head must be dipped to a sufficient depth in the specimen contained in a vessel such as a cup. In view of these drawbacks of the wet type ion selecting electrode, it has been proposed in Japanese Unexamined patent Publication No. 52(1977)-142584 to stack four functional layers on a substrate and form a film-like dry type solid ion selecting electrode. To conduct measurement with the film-like solid electrode of this type, a very small amount (e.g. between 5 .mu.l and 50 .mu.l) of a specimen is applied to a predetermined position on the ion selecting layer of the solid electrode.
FIG. 1, a solid ion selecting electrode 10 comprises a metal layer 11, a water-insoluble metal salt layer 12, a reference electrolyte layer 13 and an ion selecting layer 14 sequentially stacked as the functional layers on a substrate 19. For example, the metal layer 11 is formed of silver, the water-isouble metal salt layer 12 is formed of silver chloride, and the reference electrolyte layer 13 is made by dispersing potassium chloride in a hydrophilic organic polymer binder.
It has also been proposed in Japanese patent application No. 55(1980)-92378 to use a film-like electrode comprising three stacked functional layer, in which the reference electrolyte layer 13 shown in FIG. 1 is omitted, and the ion selecting layer 14 consisting of organic materials is directly positioned on the water-insoluble metal salt layer 12. Further, Japanese Unexamined Patent Publication No. 48(1973)-82897 discloses film-like solid electrode comprising two stacked functional layers, in which the water-insoluble metal salt layer 12 and the reference electrolyte layer 13 shown in FIG. 1 are omitted, and an ion selecting layer 14 containing an ion exchange material is directly positioned on the metal layer 11.
In case the ion to be measured is Cl.sup.- and the electrode comprises a metal layer 11 made of silver and an insoluble metal salt layer 12 made of silver chloride, it is possible to position on the silver chloride layer a halogen ion-pervious coating layer made of, for example, cellulose acetate, polymethacrylic acid, polyacrylic acid, or poly(2-hydroxyethyl acrylate) employed in a halogen ion-pervious coating layer as disclosed in Japanese Unexamined Patnent Publication 55(1980)-89741. The halogen ion-pervious coating layer is also referred to herein as the ion selecting layer.
FIG. 2 is a schematic view showing a conventional ion measuring instrument comprising two film-like solid electrodes of the type shown in FIG. 1, as disclosed in U.S. Pat. No. 4,053,381. The conventional ion measuring instrument shown in FIG. 2 comprises film-like solid ion selecting electrodes 10 of the type shown in FIG. 1, which are fixed in a frame 31 so that they are electrically isolated from each other. A porous bridge 20 formed of a porous member extends over the film-like solid ion selecting electrodes 10. A potentiometer (or a potential indicator) 51 is connected to the metal layer 11 of the electrodes 10 by lead wires 52 and 53. When measurement is conducted with the instrument shown in FIG. 2, a specimen and a standard solution are dropped almost at the same time into liquid receiving holes 28 and 29 respectively, which are perforated through the bridge 20 positioned on the electrodes 10. The specimen and the standard solution then exhibit capillary phenomena and penetrate through the porous member of the bridge 20. When the specimen and the standard solution contact each other approximately at the center of the bridge 20 and ion transfer occurs, the difference in potential between the electrodes 10 is indicated on the potentiometer 51. By measuring the difference in potential, it is possible to determine the concentration or the activity of an ion contained in the specimen.
FIG. 3 is a sectional schematic view, taken along the line S-S in FIG. 2. Japanese Unexamined Patent Publication No. 55(1980)-20499 discloses a bridge 20 having a configuration as shown in FIG. 3 comprising a non-porous bottom substrate 22 (existing nearest to the solid ion selecting electrode), an intermediate porous layer 21, and a top non-porous hydrophobic layer 24 (existing farthest from the solid ion selecting electrode). The bridge 20 is formed as a flat three-layer laminate strip having the liquid receiving holes 28 and 29, into which solutions 41 and 42 are dropped.
To prevent the functional layers of the electrodes from being short-circuited at their ends due to the specimen or the standard solution bleeding out of the bridge 20, the bridge 20 is sealed from the electrodes at least at the circumferences of the liquid receiving holes 28 and 29.
The intermediate porous layer 21 is made, for example, of porous paper, a membrane filter, threads, a fabric or the like. The layer 21 absorbs the droplets 41 and 42 and causes them to contact each other, resulting in ion transfer. When droplets are applied to the liquid receiving holes 28 and 29, the droplets fill up the holes, form a large "lid" on the top layer 24, and are then absorbed into the layer 21 in five to 30 seconds. The liquids diffuse through the bridge 20 and come into contact with each other at approximately equal distances from the liquid receiving holes 28 and 29, i.e. approximately at the center of the bridge 20. In this way, ion transfer becomes possible, and potential develops between the electrodes 10. Further, sufficient liquids to fill up the liquid receiving holes 28 and 29 are not absorbed into the layer 21 but remain in the holes 28 and 29.
Other examples of the materials preferable as the intermediate porous layer are described in Japanese Unexamined Patent Publication No. 52(1977)-142584.
The conventional ion measuring instrument described above can measure the ionic concentration or the ionic activity by use of small amounts of a specimen and a standard solution. Further, since the instrument is disposable, it does not require maintenance, cleaning and conditioning of the electrodes and thus is easy to handle. However, the conventional instrument is disadvantageous in that, because it is thrown away after being used for measurement, the expensive metals such as Ag and metal salts such as AgCl contained in the electrodes are wasted each time measurement is conducted. Further, the ion selecting layer described above consists of an organic compound (ion carrier) possessing ion selecting capability, a carrier solvent and an organic polymer binder. As the ion carrier, an ion exchange material, a crown ether compound or an antibiotic (e.g. Valinomycin capable of selecting potassium ion, or the like) is used. However, when the ion carrier such as Valinomycin is recovered together with the metal such as Ag and the metal salt such as AgCl after measurement, the ion carrier will attach to the human body during the recovering operation and adversely affects the human body.